Awards

NO. 6: I-91 Rockingham Bridges

Jan. 4, 2021

The new I-91 Rockingham Bridges are Vermont’s first four-span spliced precast concrete girder structure and is the last of three major bridge replacements on I-91 in southeastern Vermont.

The design-build project uses one-of-a-kind elements and some of Vermont’s largest precast segments, raised with massive cranes, to expedite and simplify construction while overcoming a unique unbalanced layout. The team constructed the wider, safer bridge atop a challenging topography and more than 130 ft above an environmentally critical river.

HDR’s team evaluated several bridge types. Converting the original concept to twin 852-ft-long, four-span bridges with precast, pre-stressed concrete and post-tensioned bulb tee girders, the haunched, spliced structure was the most efficient and cost-effective.

The new structure consists of five runs of seven individual concrete beams—weighing up to 93 tons each, installed on temporary framework, aligned and temporarily held in position with 30 steel strongbacks. The ends of the beams were tied together with cast-in-place concrete closures. In the end, the team installed, tensioned, and grouted three 19-strand tendons in each girder.

A unique aspect of the bridges’ design are their unbalanced spans, ranging from 167 to 245 ft. The unbalanced spans allowed the team to use the existing piers as temporary supports while erecting the structures’ haunched girders.

The new bridges were designed for a 100-year service life, 25 years longer than standard for other bridges in the area. To accomplish this, designers used stainless steel reinforcing, as well as high-performance, 9,000 psi concrete that is highly resistant to chloride penetration.

The concrete girder segments, 96 to 147.5 ft in length, vary in depth from 6 to 10 ft, and weigh up to 187,000 lb. The structure includes piers varying in height from 70 to 138 ft tall.